Project Summary Sex differences in the development and prevalence of mental illnesses are widespread, suggesting that the factors that promote disease risk and resilience may be distinct in men and women. The neurobiological mechanisms underlying this possibility are poorly understood, however, because the vast majority of pre- clinical animal research has been conducted in males. In addition, the behavioral outcome measures used to understand learned fear have focused predominantly on passive, or freezing behavior. We recently identified an active, escape-like conditioned fear response in rats (?darting?) that occurred almost exclusively in females and was predictive of improved extinction retention. A better understanding of potentially sexually dimorphic mechanisms that drive this behavior could open new avenues for treatment of disorders that are more prevalent in women, like PTSD. We propose here to define the neural circuitry that drives the selection of conditioned darting vs. freezing, and how that integrates with known extinction circuits to promote extinction retention. Our preliminary data implicate the infralimbic cortex (IL) in this role, and we will use pharmacological, chemogenetic, and viral techniques to probe how descending IL projections to the dorsal and ventral periaqueductal gray (PAG) may mediate a strategic switch between active and passive responding and drive long-term behavioral flexibility through putative collaterals in the amygdala. We will then use sophisticated machine vision and machine learning-based tools to define the broader behavioral repertoires within which darting exists, thereby identifying a multifaceted phenotype that could provide insight into the mechanisms underlying individual differences in stress resilience and vulnerability.